Tài liệu Air conditioning system design manual (ashrae special publications)

PREFACE This second edition represents a major update and revision of the ASHRAE Air-Conditioning System Design Manual. The request that drove this revision effort was simply to make a successful resource more current. The revision process involved a thorough editing of all text in the manual, the addition of SI units throughout, the updating of references, and the editing of many illustrations. New material dealing with design process, indoor air quality, desiccant dehumidification, and “green” HVAC&R systems was added. The editor acknowledges the active assistance of a Project Monitoring Subcommittee (with Warren Hahn as Chairman) from ASHRAE Technical Committee 9.1, which supervised the revision of this manual. The editor and committees are grateful to several individuals who reviewed all or parts of the draft of this revision and made valuable suggestions for improvements and clarifications (see list of contributors). Andrew Scheidt, University of Oregon, provided graphic assistance for the editing of many illustrations. Walter Grondzik, PE, Editor ix ACKNOWLEDGMENTS LIST OF CONTRIBUTORS Final Voting Committee Members Dennis J. Wessel, PE, LEED Karpinski Engineering, Inc. John L. Kuempel Jr. Debra-Kuempel Stephen W. Duda, PE Ross & Baruzzini, Inc. Kelley Cramm, PE Integrated Design Engineering Associates Rodney H. Lewis, PE Rodney H. Lewis Associates, Inc. John E. Wolfert, PE Retired Howard J. McKew, PE, CPE RDK Engineers, Inc. Phillip M. Trafton, VC Donald F. Dickerson Associates Mark W. Fly, PE AAON, INc. Gene R. Strehlow, PE Johnson Controls, Inc. Hollace S. Bailey, PE, CIAQP Bailey Engineering Corporation Lynn F. Werman, PE Self-Employed Charles E. Henck, PE, LEED Whitman, Requardt & Associates Harvey Brickman Warren G. Hahn, PE CEO, Hahn Engineering, Inc. K. Quinn Hart, PE US Air Force Civil Engineer Support Agency William K. Klock, PE EEA Consulting Engineers, Inc. John I. Vucci University of Maryland xi Other Major Contributors and Reviewers (Only major reviewers and contributors are listed. The committee is very thankful to numerous individuals who freely gave their time to review special parts of this manual.) Charles G. Arnold, PE HDR One Company Arthur D. Hallstrom, PE Trane Rodney H. Lewis, PE Rodney H. Lewis Associates David Meredith, PE Penn State Fayette, The Eberly Campus Joseph C. Hoose Cool Systems, Inc. James Wilhelm, PE Retired Paul A. Fiejdasz, PE Member ASHRAE Hank Jackson, PE Member ASHRAE John G. Smith, PE Michaud Cooley Erickson Consulting Engineers William G. Acker Acker & Associates Chuck Langbein, PE Retired A special thanks to John Smith, David Meredith, and Chuck Langbein, who reviewed and commented on each chapter through all revisions. Warren G. Hahn, PE, TC 9.1, Air-Conditioning System Design Manual Update and Revision Subcommittee Chairman xii CHAPTER 1 INTRODUCTION 1.1 PURPOSE OF THIS MANUAL This manual was prepared to assist entry-level engineers in the design of air-conditioning systems. It is also usable—in conjunction with fundamental HVAC&R resource materials—as a senioror graduate-level text for a university course in HVAC system design. This manual was intended to fill the void between theory and practice, to bridge the gap between real-world design practices and the theoretical knowledge acquired in the typical college course or textbook. Courses and texts usually concentrate on theoretical calculations and analytical procedures or they focus upon the design of components. This manual focuses upon applications. The manual has two main parts: (1) a narrative description of design procedures and criteria organized into ten chapters and (2) six appendices with illustrative examples presented in greater detail. The user/reader should be familiar with the general concepts of HVAC&R equipment and possess or have access to the four-volume ASHRAE Handbook series and appropriate ASHRAE special publications to obtain grounding in the fundamentals of HVAC&R system design. Information contained in the Handbooks and in special publications is referenced—but not generally repeated—herein. In addition to specific references cited throughout the manual, a list of general references (essentially a bibliography) is presented at the end of this chapter. The most difficult task in any design problem is how to begin. The entry-level professional does not have experience from similar projects to fall back on and is frequently at a loss as to where to start a design. To assist the reader in this task, a step-by-step sequence of design procedures is outlined for a number of systems. 1 2⏐ INTRODUCTION Simple rules are given, where applicable, to assist the new designer in making decisions regarding equipment types and size. Chapter 2 addresses the difference between analysis and design. The chapter covers the basic issues that are addressed during the design phases of a building project and discusses a number of factors that influence building design, such as codes and economic considerations. Human comfort and indoor air quality, and their implications for HVAC&R systems design, are discussed in Chapter 3. Load calculations are reviewed in Chapter 4. The specifics of load calculation methodologies are not presented since they are thoroughly covered in numerous resources and are typically conducted via computer programs. HVAC&R system components and their influence on system design are discussed in Chapter 5. Chapters 6 through 8 cover the design of all-air, air-and-water, and all-water systems, respectively. Here, again, a conscious effort was made not to duplicate material from the ASHRAE Handbook— HVAC Systems and Equipment, except in the interest of continuity. Chapter 6 is the largest and most detailed chapter. Its treatment of the air side of air-conditioning systems is equally applicable to the air side of air-and-water systems; thus, such information is not repeated in Chapter 7. Chapter 9 covers a variety of special HVAC&R systems. Controls are treated in Chapter 10. The appendices contain detailed descriptions and design calculations for a number of actual HVAC&R-related building projects. They serve to illustrate the procedures discussed in the main body of the manual. The projects in the appendices were chosen to cover a variety of building applications and HVAC system types. They help to give the entering professional a “feel” for the size of HVAC&R equipment, and they indicate how a designer tackles particular design problems. Since these examples come from actual projects, they include values (such as thermal properties, utility costs, owner preferences) that are particular to the specific contexts from which they were drawn. The purpose of the examples is to show process, not to suggest recommended or preferred outcomes. A few words of advice: do not hesitate to make initial design assumptions. No matter how far off the specific values of a final solution they might prove to be, assumptions enable the designer to start on a project and to gradually iterate and improve a proposed design until a satisfactory solution has been obtained. Frequently, more experienced colleagues may be able to assist by giving counsel and the benefit of their experience, but do not hesitate to plunge ahead on your own. Good luck! AIR-CONDITIONING SYSTEM DESIGN MANUAL⏐3 1.2 HOW BEST TO USE THIS MANUAL The following suggestions are made to obtain maximum benefit from this manual: 1. 2. 3. 4. 5. Consider the general category of the building being designed and read the appropriate chapters in the ASHRAE Handbook— HVAC Applications and the ASHRAE Handbook—HVAC Systems and Equipment to determine likely systems to consider for application to the project. Familiarize yourself with the theory and basic functions of common HVAC&R equipment. The best sources for this information are HVAC&R textbooks and the ASHRAE Handbook series. Read the chapters in this manual that address the systems of interest. Review the example problems in the appropriate appendices of this manual. Become familiar with state and local building codes, ASHRAE standards and guidelines, and applicable National Fire Protection Association (NFPA) resources. Remember that this manual, in general, does not repeat information contained in ASHRAE Handbooks and special publications. You cannot, therefore, rely on this manual as the only reference for design work. As you gain experience, make notes of important concepts and ideas (what worked and what did not work) and keep these notes in a readily accessible location. This manual is intended to point the way toward building such a design database. The best design reference available is the experience of your colleagues and peers. While an attempt has been made in this manual to incorporate the experience of design professionals, no static written material can replace dynamic face-to-face interaction with your colleagues. Use every opportunity to pick their brains, and let them tell you what did not work. Often, more is learned from failures than from successes. 1.3 UNITS The first edition of this manual was written using I-P (inchpound) units as the primary measurement system. In this edition SI (System International) units are shown in brackets following the I-P units. Conversions to SI units are “soft approximations” with, for example, 4 in. being converted as 100 mm (versus the more accu- 4⏐ INTRODUCTION rate conversion to 101.6 mm or use of a true SI commercial size increment for a given product). See the ASHRAE guide “SI for HVAC&R” (available at no cost from the ASHRAE Web site, www.ashrae.org) for detailed information on preferred measurement units and conversion factors for HVAC&R design work. 1.4 GENERAL BIBLIOGRAPHY In addition to specific references listed in each of the chapters of this manual, the following publications are generally useful to HVAC&R system designers. They should be available in every design office. ASHRAE publications are available from the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, NE, Atlanta, GA 30329-2305. ASHRAE publications are updated on a regular basis (every four years for handbooks, often more frequently for standards and guidelines). The publication dates shown below are current as of the updating of this manual but will change over time. Consult the ASHRAE Web site (www.ashrae.org) for information on current publication dates. ASHRAE Handbooks (available on CD or as printed volumes, in I-P or SI units) ASHRAE. 2003. 2003 ASHRAE Handbook—HVAC Applications. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. ASHRAE. 2004. 2004 ASHRAE Handbook—HVAC Systems and Equipment. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 2005. 2005 ASHRAE Handbook—Fundamentals. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. ASHRAE. 2006. 2006 ASHRAE Handbook—Refrigeration. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. ASHRAE Standards and Guidelines ASHRAE. 1995. ANSI/ASHRAE Standard 100-1995, Energy Conservation in Existing Buildings. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 1996. ASHRAE Guideline 1-1996, The HVAC Commissioning Process. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. AIR-CONDITIONING SYSTEM DESIGN MANUAL⏐5 ASHRAE. 2004a. ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. ASHRAE. 2004b. ANSI/ASHRAE Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 2004c. ANSI/ASHRAE Standard 62.2-2004, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 2004d. ANSI/ASHRAE/IESNA Standard 90.1-2004, Energy Standard for Buildings Except Low-Rise Residential Buildings. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 2004e. ANSI/ASHRAE Standard 90.2-2004, Energy Efficient Design of Low-Rise Residential Buildings. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. ASHRAE. 2005a. ASHRAE Guideline 0-2005, The Commissioning Process. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Other ASHRAE Publications ASHRAE. 1991. ASHRAE Terminology of HVAC&R. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. ASHRAE. 1997. SI for HVAC&R. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 1998. Cooling and Heating Load Calculation Principles. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 2002. Psychrometric Analysis (CD). Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 2004f. Advanced Energy Design Guide for Small Office Buildings. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 2005b. ASHRAE Pocket Guide for Air Conditioning, Heating, Ventilation, Refrigeration. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 6⏐ INTRODUCTION ASHRAE. 2005c. Principles of Heating, Ventilating and AirConditioning. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 2006a. Advanced Energy Design Guide for Small Retail Buildings. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 2006b. ASHRAE GreenGuide: The Design, Construction, and Operation of Sustainable Buildings. Atlanta: ASHRAE and Elsevier/B-H. NFPA Publications (updated on a regular basis) NFPA. 2000. NFPA 92A-2000, Recommended Practice for SmokeControl Systems. Quincy, MA: National Fire Protection Association. NFPA. 2002. NFPA 90A-2002, Installation of Air Conditioning and Ventilating Systems. Quincy, MA: National Fire Protection Association. NFPA. 2003. NFPA 101-2003, Life Safety Code. Quincy, MA: National Fire Protection Association. NFPA. 2005. NFPA 70-2005, National Electrical Code. Quincy, MA: National Fire Protection Association. Other Resources Climatic Data: Climatic Atlas of the United States. 1968. U.S. Government Printing Office, Washington, DC. Ecodyne Corporation. 1980. Weather Data Handbook. New York: McGraw-Hill. Kjelgaard, M. 2001. Engineering Weather Data. New York: McGraw-Hill. USAF. 1988. Engineering Weather Data, AFM 88-29. U.S. Government Printing Office, Washington, DC. Estimating Guides: Konkel, J. 1987. Rule-of-Thumb Cost Estimating for Building Mechanical Systems. New York: McGraw-Hill. R.S. Means Co. 2005. Means Mechanical Cost Data, 28th ed. Kingston, MA. AIR-CONDITIONING SYSTEM DESIGN MANUAL⏐7 R.S. Means Co. 2005. Means Facilities Construction Cost Data, 20th ed. Kingston, MA. Thomson, J. 2004. 2005 National Plumbing & HVAC Estimator. Carlsbad, CA: Craftsman Book Company. General Resources: BOMA. 2004. Experience Exchange Report. An annual publication of the Building Owners & Managers Association International, Washington, DC. McQuiston, F.C., and J.D. Spitler. 1992. Cooling and Heating Load Calculation Manual. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. SMACNA. 1988. Duct System Calculator. Chantilly, VA: Sheet Metal and Air Conditioning Contractors’ National Association. SMACNA. 1990. HVAC Systems—Duct Design, 3d ed. Chantilly, VA: Sheet Metal and Air Conditioning Contractors’ National Association. USGBC. 2005. LEED-NC (Leadership in Energy and Environmental Design—New Construction). U.S. Green Building Council, Washington, DC. (Look also for information regarding other USGBC green building certification programs.) A number of equipment manufacturers have developed HVAC design manuals and/or equipment application notes. These are not specifically listed here, in accordance with ASHRAE’s commercialism policy, but are recommended as sources of practical design and application advice. A search of manufacturers’ Web sites (for manuals or education) will usually show what is currently available (for free or for a fee). An extensive list of applicable codes and standards, including contact addresses for promulgating organizations, is provided in a concluding chapter in each of the ASHRAE Handbooks. CHAPTER 2 THE DESIGN PROCESS 2.1 DESIGN PROCESS CONTEXT There are numerous variations of the design process, perhaps as many as there are designers. To try and place the following information into a common context, the design process structure used in ASHRAE Guideline 0-2005, The Commissioning Process (ASHRAE 2005a) will be used. For purposes of building commissioning, the acquisition of a building is assumed to flow through several broad phases: predesign, design, construction, and occupancy and operation. The design phase is often broken into conceptual design, schematic design, and design development subphases. Although the majority of design hours will be spent in the design development phase, each of these phases plays a critical role in a successful building project. Each phase should have input from the HVAC&R design team. The HVAC&R design team should strive to provide input during the earliest phases (when HVAC&R design input has historically been minimal) since these are the most critical to project success, as they set the stage for all subsequent work. Design should start with a clear statement of design intent. In commissioning terms, the collective project intents form the Owner’s Project Requirements (OPR) document. Intent is simply a declaration of the owner’s (and design team’s) needs and wants in terms of project outcomes. HVAC&R design intents might include exceptional energy efficiency, acceptable indoor air quality, low maintenance, high flexibility, and the like. Each design intent must be paired with a design criterion, which provides a benchmark for minimum acceptable performance relative to the intent. For example, an intent to provide thermal comfort might be benchmarked via a criterion that requires compliance with ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy (ASHRAE 2004b), and an intent for energy efficiency might 9 10⏐ THE DESIGN PROCESS be benchmarked with a criterion that requires compliance with ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings. Design validation involves the use of a wide range of estimates, calculations, simulations, and related techniques to confirm that a chosen design option will in fact meet the appropriate design criteria. Design validation is essential to successful design; otherwise there is no connection between design intent and design decisions. Pre- and post-occupancy validations are also important to ensure that the construction process and ensuing operational procedures have delivered design intent. Such validations are a key aspect of building commissioning. 2.2 DESIGN VERSUS ANALYSIS Anyone who has taken a course in mathematics or any of the physical sciences is familiar with the process of analysis. In a typical analysis, a set of parameters is given that completely describes a problem, and the solution (even if difficult to obtain) is unique. There is only one correct solution to the problem; all other answers are wrong. Design problems are inherently different—much different. A design problem may or may not be completely defined (some of the parameters may be missing) and there are any number of potentially acceptable answers. Some solutions may be better than others, but there is no such thing as a single right answer to a design problem. There are degrees of quality to design problem solutions. Some solutions may be better (often in a qualitative or conceptual sense) than others from a particular viewpoint. For a different context or client, other solutions may be better. It is important to clearly understand the difference between analysis and design. If you are used to looking for the correct answer to a problem (via analysis), and are suddenly faced with problems that have several acceptable answers (via design), how do you decide which solution to select? Learn to use your judgment (or the advice of experienced colleagues) to weigh the merits of a number of solutions that seem to work for a particular design problem in order to select the best among them. Figures 2-1 and 2-2 illustrate the analysis and design processes, respectively. Analysis proceeds in a generally unidirectional flow from given data to final answer with the aid of certain analytical tools. Design, however, is an iterative process. Although there are certain “givens” to start with, they are often not immutable but AIR-CONDITIONING SYSTEM DESIGN MANUAL⏐11 Figure 2-1. Diagram illustrating analysis. subject to modification during the design process. For example, an owner or architect may be confronted with the energy implications of excessively large expanses of glass that had been originally specified and may decide to reduce the area of glazing or change the glazing properties. The mechanical designer may try various system components and control strategies before finding one that best suits the particular context and conditions. Thus, design consists of a continuous back-and-forth process as the designer selects from a universe of available systems, components, and control options to synthesize an optimum solution within the given constraints. This iterative design procedure incorporates analysis. Analysis is an important part of any design. Since the first step in design is to map out the general boundaries within which solutions are to be found, it may be hard to know where and how to start because there is no background from which to make initial assumptions. To overcome this obstacle, make informed initial assumptions and improve on them through subsequent analysis. To assist you in making such initial assumptions, simple rules are given throughout the chapters in this manual, and illustrative examples are provided in the appendices. 2.3 DESIGN PHASES A new engineer must understand how buildings are designed. Construction documents (working drawings and specifications) 12⏐ THE DESIGN PROCESS Figure 2-2. Diagram illustrating design. for a building are developed as a team effort. The architect usually acts as the prime design professional and project coordinator, although experienced owners and developers may deal directly with pre-selected HVAC&R consultants. The architect interfaces with the owner, directs the architectural staff, and coordinates the work of outside or in-house mechanical, electrical, and structural engineers (among other consultants). The negotiated design fees for the consultants’ work establish an economically viable level of effort. This fiscal constraint usually seriously limits the amount of time that can be allocated to studies of alternative systems or innovative approaches. The project phases outlined below are those adopted by ASHRAE Guideline 0-2005 and are those generally recognized by the architecture profession. More explicit phases may be defined for certain projects or under certain contracts. AIR-CONDITIONING SYSTEM DESIGN MANUAL⏐13 2.3.1 Predesign Phase Before a mechanical engineer can design an HVAC system, a building program must be created. This is usually prepared by the client or his/her consultants. The program establishes space needs and develops a project budget. The building program should include, but need not be limited to, the following: • • • • • • • • • The client’s objectives and strategies for the initial and future functional use of the building, whether it be a singleor multiple-family dwelling or a commercial, industrial, athletic, or other facility. A clear description of function(s) for each discrete area within the building. The number, distribution, and usage patterns of permanent occupants and visitors. The type, distribution, and usage patterns of owner-provided heat-producing equipment. The geographic site location, access means, and applicable building and zoning codes. The proposed building area, height, number of stories, and mechanized circulation requirements. The owner’s capital cost and operating cost budgets. A clear statement of anticipated project schedule and/or time constraints. A clear statement of required or expected project quality. Although some of this information may not be available before the mechanical designer starts to work, it must be obtained as soon as possible to ensure that only those HVAC&R systems that are compatible with the building program are considered. While the architect prepares the general building program, the mechanical engineer has the responsibility of developing a discipline-specific program even though some of this information may be provided by the architect or owner. The building program and use profile provided by the owner or architect and the HVAC&R systems program developed by the engineer in response to the building functional program should be explicitly documented for future reference. This documentation is termed the Owner’s Project Requirements (OPR) by ASHRAE Guideline 0-2005, and it provides the context for all design decisions. All changes made to the program during the design process should be recorded so that the documentation is always up-to-date. 14⏐ THE DESIGN PROCESS The information that should be contained in the HVAC&R system program includes • • • • • design outdoor dry-bulb and wet-bulb temperatures (absolute and coincident); heating and cooling degree-days/hours; design wind velocity (and direction) for winter and summer; applicable zoning, building, mechanical, fire, and energy codes; and rate structure, capacity, and characteristics of available utilities and fuels. Additional information regarding solar radiation availability and subsurface conditions would be included if use of a solar thermal system or ground-source heat pump was anticipated. The environmental conditions to be maintained for each building space should be defined by • • • • dry-bulb and wet-bulb temperatures during daytime occupied hours, nighttime occupied hours, and unoccupied hours; ventilation and indoor air quality requirements; any special conditions, such as heavy internal equipment loads, unusual lighting requirements, noise- and-vibration-free areas, humidity limits, and redundancy for life safety and security; and acceptable range of conditions for each of the above. An understanding of the functional use for each area is essential to select appropriate HVAC&R systems and suitable control approaches because the capabilities of proposed systems must be evaluated and compared to the indoor environmental requirements. For example, if some rooms in a building require humidity control while others do not, the HVAC system must be able to provide humidification to areas requiring it without detriment to the building enclosure or other spaces. Some areas may require cooling, while others need only ventilation or heating. This will affect selection of an appropriate system. 2.3.2 Design Phase In conventional (business-as-usual) building projects, serious work on HVAC&R system design typically occurs in the later stages of the design phase. Projects where energy efficiency and/or green building design are part of the intent or building types where HVAC&R systems are absolutely integral to building design (labo- AIR-CONDITIONING SYSTEM DESIGN MANUAL⏐15 ratories, hospitals, etc.), will see HVAC&R design begin earlier and play a more integrated role in design decision making. The design phase is often broken down into three subphases: conceptual design, schematic design, and design development. The terms schematic design, design development, and construction documents are also commonly used to describe design process subphases. The purpose of conceptual/schematic design efforts is to develop an outline solution to the OPR that captures the owner’s attention, gets his/her buy-in for further design efforts, and meets budget. Schematic (or early design development) design efforts should serve as proof of concept for the earliest design ideas as elements of the solution are further developed and locked into place. During later design development/construction documents, the final drawings and specifications are prepared as all design decisions are finalized and a complete analysis of system performance is undertaken. The schematic/early design development stage should involve the preliminary selection and comparison of appropriate HVAC&R systems. All proposed systems must be able to maintain the environmental conditions for each space as defined in the OPR. The ability to provide adequate thermal zoning is a critical aspect of such capability. For each system considered during this phase, evaluate the relative space (and volume) requirements for equipment, ducts, and piping; fuel and/or electrical use and thermal storage requirements; initial and life-cycle costs; acoustical requirements and capabilities; compatibility with the building plan and the structural system; and the effects on indoor air quality, illumination, and aesthetics. Also consider energy code compliance and green design implications (as appropriate). Early in the design phase, the HVAC&R designer may be asked to provide an evaluation of the impact of building envelope design options (vis-à-vis energy code compliance and trade-offs and/or green building intents), heavy lighting loads (i.e., more than 2 W/ft2 [22 W/m2]), and other unusual internal loads (i.e., more than 4 W/ft2 [43 W/m2]) on HVAC system performance and requirements. Questions should also be expected regarding the optimum location of major mechanical equipment—considering spatial efficiency, system effectiveness, aesthetics, and acoustical criteria. Depending upon the level of information available, the designer may be asked to prepare preliminary HVAC system sizing or performance estimates based upon patterns developed through experience or based upon results from similar, previously designed projects. Some design esti- 16⏐ THE DESIGN PROCESS mates that may be useful for a first cut are given in Table 2-1. Additional values appropriate for this design phase can be found in the ASHRAE Pocket Guide for Air Conditioning, Heating, Ventilation, Refrigeration (ASHRAE 2005c). If envelope and internal loads are reasonably well defined, peak load and rough energy calculations for alternative HVAC systems may be prepared at this time using appropriate methods for presentation to the architect and/or owner. Although they are preliminary and will change as the building design proceeds, such preliminary loads are usually definitive enough to compare the performance of alternative systems because these systems will be sized to meet the same loads. As you gain experience, you will be able to estimate the likely magnitude of the loads for each area in a building with a little calculation effort. Resources useful during this phase of design include design manuals, textbooks, equipment literature, and data from existing installations. Frequently, this type of early system evaluation eliminates all but a few systems that are capable of providing the environmental requirements and are compatible with the building structure. If the client requests it, if architectural details have been sufficiently developed, and if the mechanical engineer’s fee has been set at a level to warrant it, comparisons between construction (first) costs and operating (life-cycle) costs and the performance of different HVAC&R systems can be made in greater detail. Typically, one system is set as a reference (or base) and other proposed systems are compared to this base system. Such an analysis would proceed according to the following steps: 1. Estimate the probable capital costs of each system using unit area allowances, a rough selection of equipment, sketches of system layouts, and such tools as: • Cost-estimating manuals • Recently completed similar projects (many technical journals contain case studies that provide such information) • Local HVAC&R contractors • Professional cost estimators • Design office files • Experienced design engineers. 2. Identify the energy source or sources available and their cost per a convenient unit of energy (million Btu, kWh, therm), considering both present and anticipated costs. Determine local AIR-CONDITIONING SYSTEM DESIGN MANUAL⏐17 Table 2-1. Selected Load and Airflow Estimates for Schematic Design General: Offices: High-Rise Apartment Buildings: Hospitals: Shopping Centers: Hotels: Restaurants: Central Plants: Urban districts College campuses Commercial centers Residential centers 450 ± 100 ft2/ton for cooling loads [12 ± 3 m2/kW] 1.5 cfm/ft2 air supply— exterior spaces [7.6 L/s per m2] 0.75 cfm/ft2 air supply— interior spaces (minimum) [3.8 L/s per m2] 400 cfm/ton air supply for all-air systems [54 L/s per kW] 500 ft2/ton [13 m2/kW] based upon: lights—1.5 W/ft2 [16 W/m2] fans—0.75 W/ft2 [8 W/m2] pumps—0.25 W/ft2 [2.7 W/m2] miscellaneous electrical— 2.0 W/ft2 [21 W/m2] occupancy—150 ft2/person [14 m2/person] 1000 ft2/ton for north-facing apartments [26 m2/kW] 500 ft2/ton [13 m2/kW] for others 333 ft2/ton based upon 1000 ft2/bed [8.7 m2/kW at 93 m2/bed] average—400 ft2/ton [10.4 m2/kW] department stores—2 W/ft2 [21 W/m2] specialty stores—5 W/ft2 [54 W/m2] 350 ft2/ton [9.1 m2/kW] 150 ft2/ton [3.9 m2/kW] 380 ft2/ton [9.9 m2/kW] 320 ft2/ton [8.3 m2/kW] 475 ft2/ton [12.4 m2/kW] 500 ft2/ton [13 m2/kW]